Stratigraphy of Playas in the Pantex Area- Playa 5 and Pantex Lake

Sediments of two playas in the Pantex area, Playa 5 and Pantex Lake, were examined to (1) look for evidence of alteration that may have occurred as a result of recharge and (2) provide a context for hydrologic and hydrochemical studies of these playas. These playas are significant in that they have received large volumes of sewage and industrial and agricultural wastewater. The results of these studies, coupled with unsaturated zone hydrologic and geochemical studies, clarify our understanding of the pathways and processes used by recharging wastewater. This better understanding of recharge processes can be used to guide remediation and more quantitatively estimate the amount of contaminant remaining in the unsaturated zone. Playa sediments beneath Playa 5 are more than 70 ft thick and contain evidence of multiple episodes of lake enlargement and reduction. The stratigraphy of the older lake days is more complex than that of the younger clays, and evidence of post-depositional dip, complex lake shape, and migration of the center of the lake suggests that the evolution of the lake basin may have been influenced by syndepositional subsidence, perhaps related to dissolution of Permian salt. The steepest northwest shore of the lake shows strongly erosional relationships between upland and lake sediments, with little or no Blackwater Draw upland accretionary fades preserved. The sewage discharge pipe formerly sited in this area therefore discharged water into a pit scoured into the lower medium sand unit. These fades relationships may have influenced the movement of fluids and contaminants in this lake. Evidence of wastewater movement through sediments is recorded by organic stains and abundant limonite, and possibly by patchy reduction of sediment.